Your Samsung Galaxy Note 8 has a fast charging feature wherein the battery can be fully replenished in less than an hour from being empty or very low level. However, there are certain requirements for this feature to work and those pre-requisites are actually for the safety of the owner. So, if your Galaxy Note 8 no longer charges fast, here are the things you can do:

Enable Galaxy Note 8 fast charging feature - this feature can be turned off in fact, it's disabled by default. It must be the first thing you need to look into if your Note 8 no longer fast charges.

Turn your Note 8's screen while charging - this is one of the pre-requisites for this feature to work and it's because the phone heats faster when fast charging so if the screen is also turned on, your phone may overheat. That's the reason why fast charging is halted immediately the moment the screen lights up.

Make sure you're using the original cable and AC adapter - the fast charging feature on your Note 8, while it's integrated into the system, must be triggered by a charger with Adaptive Fast Charging capability. Your phone's charger has it but we don't know about third-party chargers that offer the same.

Turn off your phone while charging - in case you've done the first three things and the phone is still not fast charging, try turning it off and then reconnect it to its charger. When the phone is powered down, it conforms with all the pre-requisites.

Reset your phone - if everything else fails, you should try resetting your phone to bring it back to its factory default settings and then try charging it again.

Here are other things you can also try:

Check if the charger port is dry because your phone would not charge if it detects moisture in it. In fact, you'll be getting the 'moisture detected' error.

Try to see if there's an available firmware update and download it.

Try using a wireless charger.

If all else fails, you should bring the phone to the shop or back to the store so that a technician can fix it for you.